Polycarbonates (PCs) are outstanding engineering thermoplastics that have an excellent combination of properties, such as high heat distortion temperatures, low color, transparency, melt processability and outstanding toughness. These materials are used in a wide variety of applications and are produced on an enormous scale commercially. However, polycarbonates lack flame resistance, and there is a demand and need for flame resistant polycarbonates. A variety of approaches have been undertaken to impart flame resistance to these materials, but these approaches have been unsuccessful largely because flame resistant PCs lose a lot of the advantageous properties that polycarbonates inherently possess.
Polyphosphonates are known to exhibit excellent fire resistance (see for example, U.S. Pat. Nos. 2,682,522, 2,891,915 and 4,331,614). Thus, it would seem reasonable to combine polycarbonates and polyphosphonates in an attempt to combine their attributes to produce a material with the physical properties of polycarbonates and the added feature of flame resistance. However, the realization of this combination's properties by combining polycarbonates and polyphosphonates has proven to be extremely difficult.
The synthesis of random copolycarbonate/phosphonates by condensing a carbonate precursor, an aryl phosphonic acid dichloride and an aromatic diol in as halogenated solvent such as methylene chloride is described in U.S. Pat. No. 4,223,104. Using this process, random copolycarbonate/phosphonates are isolated from a solution by precipitation into methanol or by evaporation of the solvent. However, this method gives completely random copolycarbonate/phosphonates, uses expensive monomers (aryl phosphonic acid dichloride), and undesirable halogenated solvents.
In a similar manner, random copolycarbonate/phosphonates have been synthesized from the reaction of a diaryl carbonate, an aromatic bisphenol and a phosphonic acid diaryl ester in a melt condensation reaction using a basic catalyst, and in some cases a branching agent as described in U.S. Pat. Nos. 4,322,520, 4,401,802, 4,481,350, 4,508,890, 4,762,905. This synthetic method also gives completely random copolycarbonate/phosphonates that do not exhibit an acceptable combination of properties. Most notably, these random copolycarbonate/phosphonates lack toughness, and their heat distortion temperature is significantly reduced below that of polycarbonate.
U.S. Pat. No. 4,719,279 described another process wherein oligophosphonates with phenolic endgroups are first synthesized via the reaction of a phosphonic acid diaryl ester with an excess of an aromatic bisphenol using a basic catalyst. This oligomer is subsequently reacted in a phase boundary process with an aromatic dicarboxylic acid dichloride or phosgene, or a mixture of both, to give a product with less than 0.01% phenolic content by weight. This method is somewhat complex and requires the use of halogenated solvents that are environmentally unacceptable. Moreover, the resultant materials do not exhibit a favorable combination of properties with regard to toughness and other mechanical properties.
Another method, as described in U.S. Pat. No. 4,782,123, involves the preparation of copolycarboxylate/carbonate/phosphonates by the extrusion of a mixture of an aromatic polyester, a polycarbonate, and an aromatic polyphosphonate in a solvent at temperatures ranging from 150 to 420°. However, this approach also utilizes halogenated solvents that are environmentally unacceptable.
U.S. Pat. No. 4,332,921 described methods that attempt to combine the attributes of polycarbonates and polyphosphonates by making physical blends of the two via melt mixing. This method gives a physical blend of a polycarbonate and a polyphosphonate with 2 different distinct glass transition temperatures, one for PC and one for polyphosphonate. This blend is likely to be unstable under various thermal conditions and mechanical loads leading to subsequent phase segregation and consequent changes in properties.
Attempts have been made to add flame resistance to the properties of polycarbonates through the use of phosphorus containing additives, monomers, and polymers. However, none have been successful in producing a material that could be prepared in a simple and facile manner at an acceptable cost and having an acceptable combination of physical and mechanical properties.
In view of the above, there is a need for a simple, facile, cost effective method to produce copolycarbonate/phosphonates that exhibit a favorable combination of physical and mechanical properties. Therefore a method to produce such materials is disclosed in the instant invention. The compositions are comprised of block copolycarbonate/phosphonates that exhibit significant improvements in flame resistance relative to polycarbonate, with minimal sacrifice to the properties of polycarbonates such as high heat distortion temperature, low color, high toughness, hydrolytical stability, and high glass transition temperature (Tg). Further, the block copolycarbonate/phosphonates of the present invention exhibit higher Tgs, superior heat distortion temperatures, and toughness as compared to random copolycarbonate/phosphonates.